Terephthalic acid and other aromatic carboxylic acids are widely used in manufacture of polyesters, commonly by reaction with components comprising ethylene glycol, higher alkylene glycols or combinations thereof, for conversion to fiber, film, containers, bottles and other packaging materials, and molded articles.
In commercial practice, aromatic carboxylic acids are commonly made by liquid phase oxidation in an aqueous acetic acid solvent of methyl-substituted benzene and naphthalene feedstocks, in which the positions of the methyl substituents correspond to the positions of carboxyl groups in the desired aromatic carboxylic acid product, with air or another source of oxygen, which is normally gaseous, in the presence of a bromine-promoted catalyst comprising cobalt and manganese. The oxidation is exothermic and yields aromatic carboxylic acid together with by-products, including partial or intermediate oxidation products of the aromatic feedstock, and acetic acid reaction products, such as methanol, methyl acetate, and methyl bromide. Water is also generated as a by-product. Aromatic carboxylic acid, typically accompanied by oxidation by-products of the feedstock are commonly formed dissolved or as suspended solids in the liquid phase reaction mixture and are commonly recovered by crystallization and solid-liquid separation techniques. The exothermic oxidation reaction is commonly conducted in a suitable reaction vessel at elevated temperature and pressure. A liquid phase reaction mixture is maintained in the vessel and a vapor phase formed as a result of the exothermic oxidation is evaporated from the liquid phase and removed from the reactor to control reaction temperature. The vapor phase comprises water vapor, vaporized acetic acid reaction solvent, oxygen gas not consumed in oxidation, gaseous by-products such as methanol, methyl bromide and methyl acetate, carbon oxides and, when the oxygen source for the process is air or another oxygen-containing gaseous mixture, nitrogen, carbon oxides and other inert gaseous components of the source gas.
Pure forms of aromatic carboxylic acids are often favored for manufacture of polyesters for important applications, such as fibers and bottles, because impurities, such as by-products generated from aromatic feedstocks in such oxidation processes and more generally, various carbonyl-substituted aromatic species, are known to cause or correlate with color formation in polyesters made from the acids and, in turn, off color in polyester converted products. Pure forms of aromatic carboxylic acids with reduced levels of impurities can be made by further oxidizing crude products from liquid phase oxidation as described above but at one or more, progressively lower temperatures and oxygen levels, and during crystallization to recover products of the oxidation, for conversion of feedstock partial oxidation products to the desired acid product, as known from U.S. Pat. Nos. 4,877,900, 4,772,748 and 4,286,101. Pure forms of terephthalic acid and other aromatic carboxylic acids with lower impurities contents, such as purified terephthalic acid or “PTA”, are made by catalytically hydrogenating less pure forms of the acids, such as crude product comprising aromatic carboxylic acid and by-products generated by liquid phase oxidation of aromatic feedstock, in solution at elevated temperature and pressure using a noble metal catalyst. In commercial practice, liquid phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid and purification of the crude product are often conducted in continuous integrated processes in which crude product from liquid phase oxidation is used as starting material for purification.
The high temperature and pressure vapor phase generated by liquid phase oxidation in such processes is a potentially valuable source of recoverable acetic acid reaction solvent, unreacted feed material, reaction by-products and energy; however, its substantial water content, high temperature and pressure and corrosive nature due to components such as gaseous methyl bromide, acetic acid solvent and water pose technical and economic challenges to separating or recovering components of the off-gas for recycle and recovering its energy content. Further, even minor amounts of impurities that remain unseparated in recovered process streams can prevent re-use of the recovered streams if impurities adversely affect other process aspects or product quality. As described in U.S. Pat. No. 5,200,557, for example, monocarboxylic acids adversely affect hydrogenation catalysts used in purification processes, with even low levels of acetic acid residues such as present in crude aromatic carboxylic acid products recovered from oxidation reaction liquids being considered detrimental.
British Patent Specification 1,373,230, U.S. Pat. Nos. 5,304,676; 5,723,656; 6,143,925; 6,504,051, European Patent Specification 0 498 591 B1 and International Application WO 97/27168 describe processes for manufacture of aromatic carboxylic acids by liquid phase oxidation of aromatic feed materials in which a high pressure off-gas is removed from the oxidation and treated for recovery and recycle of portions or components thereof and, in some cases, recovery of energy. Water recovered from high pressure oxidation off-gases in these processes is generally returned to oxidation with acetic acid condensed from off-gases, recycled to off-gas separations for use as reflux or disposed of in liquid purge streams. Water condensed from oxidation off-gas and then purified by distillation is used to wash a purified terephthalic acid precipitate according to embodiments of U.S. Pat. No. 5,304,676 and treated water condensed from an expanded, low pressure gas after separation of monocarboxylic acid and water vapors in a high pressure oxidation off-gas, catalytic oxidation of a resulting high pressure gas comprising water vapor to remove organic impurities by conversion to water and carbon oxides and expansion of the resulting gas to recover energy is used as a crystallization solvent for purified terephthalic acid according to embodiments of U.S. Pat. No. 5,723,656. However, none of the processes uses liquid condensed from a high pressure off-gas from a liquid phase oxidation as solvent or other liquid comprising water in the purification of impure aromatic carboxylic acids. Further, recoveries of materials and energy from off-gases in such processes often are accomplished at the expense of each other, for example due to loss of energy content on cooling or depressurizing to recover materials, burning of materials to control atmospheric emissions and other losses of oxidation solvent, feedstock and intermediates that result if the high temperature and pressure vapor phase resulting from oxidation is not cooled or depressurized for removal of such materials. Impurities remaining in recycle streams can upset process operation and impair product quality. Added equipment and process steps for recovering materials, energy or both can add further process complexities and limit or preclude their practical utility if they add costs that outweigh materials and energy savings.
Impact of lost energy and materials are magnified by scale of process operations. In world-scale commercial manufacturing plants with annual capacities of 500,000 to 1,000,000 or more tons of product, even fractional percentages or hundreds of parts per million of feedstock and solvent lost or converted to undesired or unusable by-products, minor inefficiencies in energy recovery and incremental additions to effluent water treatment translate to significant practical losses of materials, increases in consumption of fuel or electricity and added processing, as well as unpredictable process efficiencies and economics due to differences and variations in costs for energy, materials and requirements for treatment of gaseous and liquid emissions and effluents.